Internal fault detection of electrical machines as described in the following invention is a technique for rapid electronic detection of electric power faults or malfunctions in rotating and linear electric machinery particularly to the primary winding. These type of faults are generally due to aging effects of the primary winding, mechanical damage to the primary winding or an overvoltage/overcurrent electrical condition which accelerates the winding insulation degradation. The primary windings may be composed of conductor of the normal conducting type or the superconducting type employing advanced materials such as niobium-titanium, niobium-tin, yttrium or bismuth alloys. The machine faults which are most prevalent are categorized into eight distinct types:
a. Line to ground coil insulation faults within the primary slot resulting in a short-circuit or over-current condition or overheating of the coil and magnetic core. (Type III).
b. Line-to-line coil insulation faults within the primary slot resulting in an overcurrent/overheating condition. (Type II).
c. Line-to-line coil insulation failures or short-circuits in the machine end-region or terminal box area resulting in an overcurrent/overheating condition. (Type VI).
d. Primary magnetic core faults consisting of short-circuited laminations or degradation of the interlaminar insulation resulting in conduction between laminations. (Type VII).
e. Turn-to-turn insulation faults within a particular primary coil due to insulation failure or damage. (Type I).
f. Open-circuit fault whereby conductors separate within a particular primary coil interrupting the flow of current. (Type IV).
g. Open-circuit fault whereby conductors or coil groups separate in the machine end-region or terminal box area interrupting or altering a normal flow of phase current. (Type V).
h. Partial discharge or electric dielectric failure of the primary coil insulation system resulting in an increased progressive degradation of the primary insulation system and not necessarily resulting in a short-circuit current condition. (Type VIII).
The other aspect of the described invention is the method and apparatus for segregating or isolating selected sections of primary phase windings or regrouping the primary winding to minimize the affects of a primary electrical fault on the overall operation of the motor or generator. The invention uses high speed electronic switching devices which are connected to either individual primary coils or to primary phase groups which are responsive to signals from the master control system. This system performs both a diagnostic function to determine the location and type of fault and then proceeds to determine a schedule of switching of primary coil members to allow for isolation of the electrical fault section(s) and to enhance a specific output of the electrical machine such as torque, voltage output or reactive power.
A particularly novel aspect of the described invention is the ability to segregate primary coil groups in a fashion that results in the airgap magnetic flux being maintained in a symmetrical electromagnetic condition whereas otherwise the location and magnitude of the fault would cause a large asymmetry in airgap flux spatial distribution and magnetic core magnetic flux resulting in overheating or unacceptable magnetic overloads. The criteria for performing selective coil isolation is prescribed by the master control system and dependent on the machine's specific design, output/input power level, speed and fault classification. An important aspect of the described invention is the ability of the control system to rapidly categorize the fault according to severity and type within 10-20 cycles of line frequency period time and to initiate very rapid switching (within for example 0.5 second) of the sectionalizing switches to minimize the buildup of short circuit current or overheating effects. In this fashion, the fault current does not build up to full value for a significant amount of time and effects such as prolonged arcing or overheating are entirely mitigated.
A further novel aspect of the described invention is the ability of the control system to attenuate acousto-magnetic noise within the electrical machine by virtue of having an active control means to yield symmetric magnetic airgap flux and symmetric magnetic core flux over the entire speed/torque range when significant primary winding faults are present. In a normal machine without internal fault detection, primary faults results in large unbalanced magnetic pull and accompanying acoustic-magnetic noise. In the described invention, the control system specifically switches out selected primary coils or coil groups to ensure a balanced magnetic condition and thus minimize both unbalanced magnetic pull (rotor to stator radial pull) in the machine and attenuate any sources of magneto-acoustic noise.
By maximizing the machines magnetic symmetry through fault diagnostics and subsequent faulted-coil switching, the machine's torque pulsations are attenuated and the overall acoustic performance of the entire drive is enhanced when only a limited portion of the primary core is excited in the fault mode.
The system has application to four broad types of electrical machinery:
a. synchronous rotating machinery PA1 b. asynchronous rotating machinery PA1 c. synchronous linear machinery PA1 d. asynchronous linear machinery PA1 1. permanent magnet (PM) field machines PA1 2. wound-field machines PA1 3. reluctance type machines PA1 1. solid-rotor, non-wound secondary member PA1 2. cage rotor secondary member composed of discrete conductors which are short circuited at each end of rotor and interspaced with ferromagnetic core material PA1 3. wound-rotor composed of insulated conductive secondary windings which are brought out to a set of polyphase current collectors or slip rings for connection of an external impedance or power source.
Within synchronous machinery, there are three classes as follows:
Within each class, the machines are configured with the field structure rotating (category 1) or the field structure stationary (category 2).
With the asynchronous or induction machinery, there are three broad classes as follows:
While the standard configuration for an induction machine is to have the secondary member rotating, it is understood that induction machinery can be configured with the secondary member stationary and the primary member rotating whereby the primary windings are attached to rotor slip-rings for transfer of electrical power into or out of the machine. The primary member in all 3 classes of induction machines are characterized by cylindrical ferromagnetic cores with uniform longitudinal slotting extending the length of the machine's "active core" and fitted with conductive windings which are insulated from the primary core. The described invention is applicable to what is referred to in the art as diamond shaped coils, either lap wound or concentric wound. Standard machines have double layer coils in that each primary core slot contains a coil side for upper and lower slot portions. Alternate arrangements have slots which contain 3 or 4 coil sides and also subject of this patent.
FIG. 1 shows a cross section of a 4 pole multi-megawatt wound field synchronous machine as may be used for either motor or generator application. The stator contains 54 uniform slots and the rotor contains 24 slots which are distributed uniformly so as to yield a sinusoidal spatial variation in magnetic flux along the machine bore periphery. The rotor corresponding to this type of machine is shown in FIG. 2. which has air-cooled rotor slots. The slots contain 12 conductors wound in series connection in Slot "A" and 16 conductors in series in Slot "B". The subject machine has a total of 8 Type "A" slots and a total of 16 type "B" slots.
FIG. 3 shows a cross-section of a radial flux permanent magnet machine as designed for the range of 100 kVA to 5000 kVA, either motor or generator. Permanent magnet machines are the one class of machinery which do not have the ability to instantly switch off the excitation source when a fault occurs and are consequently and inherently the most susceptible to primary winding damage when a fault occurs if there is no mechanism by which to reduce or cutoff field excitation in an expeditious fashion such as switching-off a solid state device or regulator. The permanent magnet machine must without the described invention simply wait until the rotor is physically stopped from spinning to yield zero effective excitation of the primary. Depending on the inertia of the rotor this may take several seconds minutes to several hours for the rotor to stop, during which time considerable heating of the primary windings or core is apt to exist.
The PM machine secondary or excitation structure contains axial slots which contain hard permanent magnet material such as neodymium-boron-iron or samarium-5-cobalt magnets. The energy product for these magnets are high in the range of 35-45 M-G-Oe. The stator or primary for this PM machine contains an array of axial slots in a laminated ferromagnetic core whereby the bore diameter is kept small to correspond to a small diameter rotor designed for very high rotor tip speeds such as 650-750 ft.sec. and high shaft speeds typically in excess of 20,000 rpm.
FIG. 4 shows an alternative arrangement of an axial flux permanent magnet synchronous machine whereby the orientation of the flux originating from the permanent magnets is directed axially or transverse in contrast to the machine of FIG. 3 which uses radial magnetic flux orientation. The advantage of this design is that it uses a hollow drum rotor in contrast to cylindrical rotor and in doing so has a reduced mass and inertia for the rotor with an accompanying faster speed of response. The construction of the stator core and the primary windings is materially different in this machine. The flux densities for the axial flux machine are approximately the same as with conventional radial flux machines, however the machine is generally characterized by the absence of ferromagnetic core material on the rotor which is typically restricted to an electrically conductive material. These machines are an improvement over the prior art because the length of the rotor magnetic circuit and consequent iron core losses/overall weight are significantly lower in the axial flux units.
FIG. 5 shows a winding diagram for a 6 phase, 32 pole permanent magnet naval propulsion generator which is wound specifically for use with direct rectification of each winding and phase of the machine output. The field is rotating and composed of 32 permanent magnet assemblies typically composed of neodyumium-boron-iron magnets. The flux from these magnets may be directed either radially or axially. This machine has by way of example 24 coils per armature winding arranged as 4 parallels per armature. Each stator coil is multipolar and spans 8 pole pitches. The winding and construction details are listed in Table 5.
This type of machine is non-conventional in that it is wound in peripheral sectors such as four quadrants where each quadrant contains a complete phase grouping and independent rectification means connected to each phase winding in each sector. FIG. 5a shows an arrangement whereby each sector phase winding feeds an H bridge rectifier, which may be either a diode rectifier or a controlled rectifier such as a thyristor or insulated gate bipolar transistor (IGBT). Thus the whole machine has a total of 24 H-bridge rectifiers designated blocks 231 through 254. The outputs of the rectifiers may be configured in parallel or in series or a combination thereof; the FIG. 5a shows a parallel grouping of outputs designated DC1, DC2, DC3, DC4.
FIG. 5b shows a variation on the basic machine winding as described in Table 5 with the modification of an interconnected rectifier bridge 345, 347, 349, 350 linking all phases of a * particular quadrant. This arrangement reduces the total number of active devices (diodes or thyristors) in the system and permits connecting quadrants in series or in parallel. Both FIG. 5a and 5b represent systems which are prior-art and do not have inherent quadrature or diametrical magnetic symmetry. These arrangements are sources of high magneto-acoustic noise and vibration when one coil group is opened or short-circuited or allowed to have an MMF or current loading substantially different from adjacent coil groups. However, these configurations can be specially controlled to yield magnetic symmetry and low acoustic noise by insuring that all H bridge rectifiers are controlled rectifiers along with a comprehensive control strategy for balancing of MMF along the machines periphery. For example, if coil number 2 is found to be defective, then the controlled rectifiers operating coils 2 and 14 must be made open circuit. If the acoustic-magnetic noise is not sufficiently low with diametrical symmetry, then the IFD control system so described throughout this invention must then proceed to produce quadrature magnetic symmetry by additionally having controlled rectifiers for coils 8 and 20 switch these circuits to open circuit conditions or otherwise isolate these coils from excitation derived from either airgap flux or line power.
The subject invention as applied to a permanent magnet generator modifies the arrangement of FIG. 5 by having diametrical symmetry as shown in FIG. 6. In FIG. 6a, windings 1 and 13 are permanently connected in series, winding 2 and 14 are in series, etc. continuing for each phase group and typically each phase group is floating above ground. There are two main output busses: DC1 and DC2 which may be connected in either series or parallel. Rectifiers 331 through 342 are diode or controlled rectifier H-bridges with the circuit shown in the enlarged view in FIG. 6a. This also shows the fault detection system instrumentation points for each winding as designated P1 through P12. Non-contacting current transformers are included in each phase winding and designated 351 through 362.
FIG. 6b shows a generator winding with diametrical magnetic symmetry subject of this invention with the provision of two interconnected polyphase bridge rectifier assemblies 345 and 347 as appropriate for high power applications. This has diametrical magnetic symmetry and two DC output busses: DC1 and DC2 and fusible devices F1 through F12. The fault diagnostic system uses instrumentation points P1 through P12 and current transformers on each phase winding designated 351-362.
These windings use the same windings as given in Table 1 with the exception that the coil number of turns is one-half or 8 turns per coil and the number of strands are increased from 20 to 40 strands per coil for the same terminal voltage and power rating as for FIG. 5. It is implicit in both FIG. 6a and FIG. 6b that the phase winding coils are interconnected such that they span a multiple slot pitches (e.g. 8 slots) and are electrical phased 30 degrees apart between adjacent numbers coils such as number 6 and 7. Coils numbered 1 and 13 and similar groups are thus phased 12.times.30 degrees or 360 electrical degrees apart and spaced 180 mechanical degrees apart.
The direct voltage measurements on the machine coils (P1-P12) may in the preferred embodiment for the generator be augmented by airgap radial flux sensing, stator bore mounted flux coils which span a stator slot pitch and are a full slot length. These bore mounted flux coils are made to be a slot pitch wide independent of the width (span) of the power coils in the stator winding. The flux coils then forms the major input signal to the digital signal processors comprising the internal fault detection system.
TABLE 1 ______________________________________ High Frequency PM-Rotor Generator Design Characteristics ______________________________________ Stator Output @ 85% P.F. 10 Megawatts Apparent Power Output 11.76 MVA Output Voltage/Current 600 V rms/3266 A rms No. of Phases 6 No. of Poles 32 Magnet Material Neodymium-Boron-Iron Shaft Speed 3600 rpm Output Frequency 960 Hz Parallels per Phase 4 No. of stator slots 192 Slots per rotor pole 6 Slots per pole and phase 1 Stator slot depth 2.134 cm Stator slot width 0.790 cm Stator slot area 1.686 sq. cm. Stator slot pitch 1.37 cm No. of turns per coil 8 Coils per slot 2 Current per phase parallel 816 A rms Current per slot at full load 1632 A rms/2307 A pk Current Loading at stator bore 119,124 A/m periphery No. of Litz wires/coil 8 No. of strands per Litz wire 20 strands, No. 22 gauge Cross section area of a wire 0.649 sq. mm. Cross section of a coil 103.8 sq. mm. Current density 7.85 A/sq. mm. Stator core length 52 cm Stator Lamination thickness 3 mils Stator core outer diameter 83.5 cm Stator core inner diameter 76.7 cm Radial Airgap per side 1.30 cm Airgap Magnetic field at stator bore 0.455 Tesla average Airgap Field .times. Current Loading Product 54,201 T-A/m ______________________________________